Gardening Advice: When a Tree is Too Close to Home

Q: We recently bought a big Victorian house with a huge cedar tree growing next to a two-room addition at the back. The foundation is poor (sitting on the dirt), and the roots of the tree are lifting it so that the rooms (an office and a laundry) are tilted and have begun to smell of mildew. We’ve been advised to either remove the cedar or tear down the rooms. What a choice! — Linda Altaman, Tuolumne, Calif.

A: A mature cedar like yours is priceless. That stately tree and your fine old home have aged gracefully together. Although an appraiser might say the cedar is worth only $10,000, you won’t be able to get another one like it in your lifetime for any amount of money. The newer addition, from your description, has already suffered some damage, and that’s where I think you should focus your efforts. Certainly, cutting down the tree won’t fix the addition. After spending $1,500 to have the tree removed, you’ll still have a badly listing building. The roots underneath will have to be extracted, or they’ll invite termites. The disturbance caused by removing the roots may also cause the foundation to crack, if it hasn’t already. Since a foundation repair seems unavoidable, I suggest you work around the tree.

Your first step should be to hire someone who can evaluate the tree’s health and advise you on how much of the root system can be safely cut away. In the phone book, look for a “consulting arborist.” You don’t want a “certified arborist” — they tend to be tree cutters and their solutions will lean toward massive pruning and removal. A consulting arborist will be happy to just give advice, in person and in writing. Fees run $75 to $100 per hour for consultation, and $50 to $60 per hour for excavating and root cutting or for removing a tree. Once you have the arborist’s report, you can begin talking to a builder or an architect about how to deal with the building. If the foundation needs an overhaul, you might find it’s possible just to move the addition a bit so that it’s farther from the tree. Or the building might be set on piers, to raise the floor slightly so that the roots can continue to grow freely underneath. But if the slab has not been ruined and can safely be restored to a level position, the offending roots can probably be cut away, with the arborist’s help. In many cases, pruning off a major root shouldn’t do permanent damage. Trees lose more than 90 percent of their roots when they’re transplanted and eventually recover. The roots lost on a transplanted tree, however, are those farthest from the trunk, and they’re relatively small. For a large tree, don’t make any cuts closer than 5 to 8 feet from the trunk.

The arborist should be able to prune the root for you, or at least oversee the work. The root must be exposed without damaging it, which is done by spraying water or air around the root collar; a cut is then made at a point where the root branches. To prevent roots from growing back under the slab, the arborist can install a heavy plastic root barrier about 3 feet deep in a trench along the building wall. Of course, the arborist may discover that your fine-looking tree actually has serious internal decay. If it’s in danger of breaking apart in the wind or blowing over, that poses a serious risk to you and your neighbors’ homes. If the arborist recommends removing the tree, ask him to suggest a replacement species and help you find a good specimen. Buy as mature a tree as you can afford, and locate it the proper distance from the building — half the estimated mature height of the tree is a good rule of thumb. For a large tree such as a cedar, about 40 feet from the building is good. For Japanese maples, ‘Bradford’ pears, and other medium-size trees, about 20 feet away is safe. Trees often look best placed to the side of a building rather than directly in front. They provide better shade in summer when placed on the west or east side of the house, rather than on the south. Whatever you do, resist the temptation to compensate for the new tree’s much smaller size by planting it closer to the house.

Free NewsletterSign up for weekly gardening inspiration and design tips